1. Field of the Invention
This invention relates to a spark plug and a method of making the same.
2. Description of the Related Art
Conventionally, spark plugs have been known which have a sealing material layer composed mainly of talc filled in a space between the outer face of the insulator and the inner face of the metal shell so as to seal the space for checking gas leakage from a combustion chamber. The spark plug is exposed to high temperatures and high pressure because of influences by combustion gas generated in a combustion process within the combustion chamber, and sometimes served under severe circumstances receiving vibrations, and therefore the spark plug is demanded to meet needs for completing performance under such circumstances, and in particular it is desirable to sufficiently secure a sealing property in a sealing material.
Of late, direct injection of gasoline or lean-burn system has extensively been progressed as instruments for realizing high output and low fuel combustion. Such an engine is apt to enlarge a valve diameter or bring valve position nearly to a plug hole at a center of the cylinder head, and a demand for making the spark plug small size has been arisen to reduce the diameter thereof as possible. Practically, a distance between two parallel opposite faces of a tool engaging portion for attaching the engine for fitting such as a wrench is conventionally 16 mm or more, and it has been required to reduce from 16 mm to less than 16 mm as 14 mm. While satisfying the demand for miniaturization, it has been called for to provide a spark plug considered to have a sealing property (anti-looseness) and impact resistance.
Accordingly, it is an object of the invention to provide such a spark plug having a sealing material layer, enabling to secure an excellent sealing property under circumstances at high temperature, using powder of talc as a main component. In particular, it is an object to provide a miniaturized spark plug excellent in the impact resistance and the sealing property and a method of making the same.
For solving the above mentioned problems, the invention is to provide a spark plug having a center electrode, an insulator provided around the center electrode, a metal shell provided around the insulator and a ground electrode disposed in opposition to the center electrode so as to form a spark discharge gap, and having a sealing material composed mainly of talc filled in a space between the inner face of the metal shell and the outer face of the insulator so as to seal the space, characterized in that a packing density of the sealing material is 1.5 g/cm3 to 3.0 g/cm3.
If the sealing material layer is filled in the space between the inner face of the metal shell and the outer face of the insulator such that a packing density of the sealing material is 1.5 g/cm3 to 3.0 g/cm3, compressibility of the sealing material is remarkably improved and the sealing property of the sealing material layer is heightened. Thus, if being exposed to severe using conditions to generate load in the sealing material layer by vibration, pressure and others, air tightness can be well secured between the metal shell and the insulator. Especially in the spark plug, when taking, as a front side, a side where the spark discharge gap is formed, a rear-side circumferential part of the metal shell defines a press-fitting portion facing outside, deterioration is difficult to happen even at high temperature and high pressure depending on the above mentioned sealing material layer, and the press-fitting portion can be usefully controlled from loosening to heighten the sealing property.
In the spark plug, when a distance between two parallel opposite faces of the tool engaging portion (called as xe2x80x9copposite side sizesxe2x80x9d hereafter) to be formed 6 in the metal shell for attaching the engine is W, W less than 16 mm, an inner diameter DS of a portion surrounding the sealing material layer in the metal shell satisfies 9.0 mm less than DS less than 13.0 mm, and when an outer diameter of a portion surrounded by the sealing material layer in the insulator is DI, DSxe2x88x92DI greater than 1.6 mm and DIxe2x89xa77.0 mm, and it is desirable that the filling density of the sealing material is 1.5 g/cm3 to 3.0 g/cm3.
In the small sized spark plug, the metal shell and the insulator are required to reduce diameter. Specifically, the opposite side sizes are required to be less than 16 mm. On the other hand, from the viewpoint of mechanical strength of the spark plug, the size reduction of the insulator is limited to keep enough strength. It is accordingly assumed that the sealing material layer is not furnished between the metal shell and the insulator, and such a spark plug is structured to have a large diameter of the insulator. However, the spark plug designed not to have a sealing material layer is involved with problems that the impact resistance is weak and the air-tightness is considerably lowered after giving impact. Such problems are remarkable in a spark plug, wherein the opposite side sizes of the tool engaging portion are less than 16 mm, because inevitable lack of thickness of the metal shell decreases strength thereof.
In the small sized spark plug of the opposite side sizes being less than 16 mm, the sizes of the insulator and the metal shell are determined as mentioned above (as in a second invention), and the sealing material layer is furnished between the metal shell and the insulator so as to moderate impact to the metal shell, effecting as a buffer, thereby enabling to realize a structure satisfying the impact resistance and the air-tightness. Especially, if furnished with the sealing material layer where the filling density is adjusted to range 1.5 g/cm3 to 3.0 g/cm3 difference in diameter between the inner face of the metal shell and the outer face of the insulator is shortened in comparison with conventional ones, and even in the small sized spark plug limiting the amount of the sealing material layer, the structure having excellent impact resistance and air-tightness can be realized.
A miniaturization of the spark plug reduces the difference in diameter between the inner face of the metal shell and the outer face of the insulator, and by making DSxe2x88x92DI greater than 1.6 mm in said difference, it is possible to pack the sealing material layer uniformly and at a proper density (the filling density of 1.5 g/cm3 to 3.0 g/cm3) in the gap between the metal shell and the insulator. If the difference of DSxe2x88x92DI is less than 1.6 mm, when filled with powder, the difference is too small to be filled with the sealing material layer On the other hand, when a shaped body (ring), which is preliminary shaped with powder, is disposed (filled) in the space between the metal shell and the insulator, the thickness of the ring should be less than 0.8 mm. However, the difficulties in forming thin ring brings may result in lower strength. Further, if the outer diameter DI of the insulator is less than 7.0 mm, insufficient strength thereof results in inferior function of the spark plug. On the contrary, if being DI greater than 7.0 mm, enough strength can be given to the insulator.
By the way, in the spark plug as mentioned above since it is structurally difficult to make thickness of the metal shell (actually, thickness of the tool engaging portion) larger than necessary. Therefore, if the filling density is larger than 3.0 g/cm3, high pressing pressure should be applied when the sealing material layer is filled. Such high pressure may causes deformation of the tool engaging portion, which results in deviation from tolerance. Therefore, it is desirable that the filling density of the sealing material layer is 3.0 g/cm3 in the above size determination (that is, W less than 16 mm, 9.0 mm less than DS less than 13.0 mm, DSxe2x88x92DI greater than 1.6 mm and DI greater than 7.0 mm). Thus, if the filling density of the sealing material layer is 3.0 g/cm3 or lower, even in the miniaturized spark plug difficult to make thickness of the metal shell large, the filling density can be increased as limiting deformation of the metal shell within tolerance, resulting in high precision. The opposite side size W is desirably 12 mm or more to keep sufficient strength.
The invention is further concerned with a method of making the above mentioned spark plug, comprising
a filling process for forming powder filled layer by locating the insulator inside of the metal shell and filling powder of sealing material being main of talc in the space between the metal shell and the insulator,
a compression process for compressing the powder filled layer under the above mentioned condition so as to form the sealing material layer, and
a forming process for, prior to the filling process, forming the filled powder in ring shape corresponding to said space,
wherein, in the above filling process, the formed body of the filled powder is located in said space, and in the compression process, the formed body as the powder filled layer is compressed at higher pressure than that in the forming process, whereby the sealing material layer is formed having a filling density of 1.5 g/cm3 to 3.0 g/cm3.
If performing the forming process for, prior to the filling process, forming the filled powder in ring shape corresponding to said space, raw material powder of a fixed amount can be charged easily and exactly into a narrow space between the metal shell and the insulator, contributing to heightening of production efficiency
It is desirable that, prior to carrying out the forming process, to adjust in advance average diameter of talc powder ranging 30 to 200 xcexcm, and apparent density of the talc powder to be 0.5 g/cm3 to 1.3 g/cm3. Namely, it is recommended to use the talc powder adjusted to be in this range in the forming process. By adjusting the apparent density, the ring shaped body composed of mainly the talc powder can be formed with adequate strength, and therefore the sealing material layer can be provided with an appropriate density.
If the apparent density is less than 0.5 g/cm3, the ring shaped body may be short in strength, and consequently, it is difficult to form the sealing material layer with the enough filling density and with the uniform density. On the other hand, if exceeding 1.3 g/cm3, the pressing pressure must be large when filling the sealing material layer (the formed body), resulting in that the tool engaging portion will be probably deformed by said pressing pressure as deviating from tolerance. Further, if the sealing material power adjusted ranging 30 xcexcm to 200 xcexcm, the apparent density can be determined to be precisely high. If the average diameter is less than 30 xcexcm or more than 200 xcexcm, it is difficult to provide a suited apparent density. The average diameter is desirably 80 to 150 xcexcm.
Actually, it is possible to equip a raw material powder producing process for mixing the talc powder adjusted in the above range and a binder as well as a filling powder material producing process for producing the filling powder as adjusting the raw material powder to be predetermined diameter. The sealing material layer is composed of the sealing material powder. These procedures will be mentioned in detail later.